N <- 100000
n <- 9

set.seed(123)
system.time(resRepl <- replicate(N, mean(rexp(n))))

str(resRepl)

set.seed(123)
resLoop1 <- numeric(N)        # preallocate the result of correct size
system.time(for (i in 1:N) resLoop1[i] <- mean(rexp(n)))

all.equal(resRepl, resLoop1)

set.seed(123)
resLoop2 <- numeric()                   # empty numeric vector
system.time(for (i in 1:N) resLoop2[i] <- mean(rexp(n)))

all.equal(resLoop2, resRepl)

set.seed(123)
resLoop3 <- NULL                        # empty list
system.time(for (i in 1:N) resLoop3 <- c(resLoop3, mean(rexp(n))))

str(resLoop3)

set.seed(123)
str(MM <- matrix(rexp(n*N), nr = n))

set.seed(123)
system.time(resColMeans <- colMeans(matrix(rexp(n*N), nr = n)))

all.equal(resColMeans, resRepl)

resApply <- apply(MM, 2, mean)
all.equal(resApply, resRepl)

set.seed(123)
system.time(resApply <- apply(matrix(rexp(n * N), nr = n), 2, mean))

set.seed(123)
system.time(resSapply <- sapply(1:N, function(i) mean(rexp(n))))

set.seed(123)
str(unlist(lapply(1:N, function(i) mean(rexp(n)))))

system.time(unlist(lapply(1:N, function(i) mean(rexp(n)))))

set.seed(123)
str(vapply(1:N, function(i) mean(rexp(n)), 1))

system.time(vapply(1:N, function(i) mean(rexp(n)), 1))

replicate

library(rbenchmark)

fRepl <- function(n, N)
    replicate(N, mean(rexp(n)))
fLoopPre <- function(n, N) {
    ans <- numeric(N)
    for(i in seq_len(N)) ans[i] <- mean(rexp(n))
    ans
}
fColMeans <- function(n, N)
    colMeans(matrix(rexp(n * N), nr=n))
fApply <- function(n, N)
    apply(matrix(rexp(n * N), nr=n), 2, mean)
fSapply <- function(n, N)
    sapply(integer(N), function(...) mean(rexp(n)))
fLapply <- function(n, N)
    unlist(lapply(integer(N), function(...) mean(rexp(n))))
fVapply <- function(n, N)
    vapply(integer(N), function(...) mean(rexp(n)), 1)
N <- 1000

benchmark(fColMeans(n,N),
          fVapply(n, N),
          fRepl(n, N),
          fLoopPre(n, N),
          fApply(n, N),
          fSapply(n, N),
          fLapply(n, N),
          columns = c("test", "elapsed", "relative", "user.self", "sys.self"),
          order = "elapsed")